The Italian senate has been left in turmoil after Silvio Berlusconi's lawmakers staged a walkout before a crucial reform vote, undermining support for the prime minister, Mario Monti. Photograph: Guiseppe Lami/ANSA

Italy's politics were thrown back into turmoil on when Silvio Berlusconi's party members walked out before a crucial vote in the Italian senate on Thursday, threatening the survival of Mario Monti's reforming, non-party government.

The boycott came after Berlusconi declared the government's policies had brought Italy to "the brink of the abyss". In a statement issued late on Wednesday, the 76-year-old TV and property magnate-turned-conservative politician suggested he would have to return to save his country.

As investors took fright, the yield on Italy's benchmark sovereign bonds shot up. The yield moves in the opposite direction to the markets' confidence in the country's ability to repay its debts.

The walkout came just as the upper chamber was to pass a package of measures designed to revive growth in Italy's shrinking economy. Enough senators from Berlusconi's Freedom People (PdL) movement stayed behind to ensure a quorum for the bill.

The boycott was in part a reprisal for a remark by the economic development minister, Corrado Passera, warning of the consequences of a Berlusconi comeback. But the PdL's leader in the upper house, Maurizio Gasparri, said the move signalled "the passage of our group to a position of abstention in relation to the government".

If that remains the position, it means Monti's technocrats have lost their majority in the upper house. The former EU commissioner's non-party cabinet took office in November 2011 after Berlusconi lost his majority in the lower house, the Chamber of Deputies. A general election to replace it is due before the spring.

Until now Monti has been able to count on the three-way support of the PdL, the centre-left Democratic party and a conservative Christian Democrat group. Yet, according to Berlusconi's latest declaration, the policies his own followers endorsed were ruinous for Italy.

In his statement he said unemployment was up by a million; public debt had increased; disposable incomes had collapsed and taxation was at "intolerable levels". Responding to a news agency report that had quoted him as telling his lieutenants he was not going to run in the next election because no one wanted him to, he said: "The reality is the opposite."

A headline on the website of Berlusconi's family's newspaper, Il Giornale, declared: "Berlusconi returns to the field". The billionaire politician's populist policies have always had great electoral appeal, but his party is languishing in the polls with only around 16% of the vote.